There are several types of sights utilized with small arms, such as rifles, shotguns, hand guns, bows, and other similar, hand-held weapons.
Perhaps the simplest type is the iron sight consisting of a small notch on the near end of the weapon and a bar on the far end of the weapon. When aiming, the notch, the bar, and the target must all be aligned. There are several drawbacks to this simple sight. The shooter must shift his/her focus between the sight and the target, which is difficult, particularly when the shooter or the target is moving. The notch and bar also partially block the targets seen. And, under low ambient light conditions, these sights are difficult to use due to the reduced depth of focus of the eye.
Another common sight is the telescopic sight. With this sight, the image is magnified and a reticle pattern is superimposed on the image scene at an intermediate image plane. Primarily used for aiming at distant stationary targets, telescopic sights are not suitable for use with moving targets or when the shooter is moving, because of the small field of view and exit aperture of the sight.
Another commonly used sight is the reflex type. This sight utilizes an illuminated reticle and a lens that collimates every point of the reticle to project the reticle pattern to the target plane. A partially reflecting mirror or beam combiner is located downstream of the collimating lens to superimpose the reticle pattern over the real world scene. The exit aperture is the same size as the collimating lens, but is recessed further from the viewer. The eye position from which the reticle can be seen is thus restricted by the lens diameter and the recessed exit aperture. Also, these sights are bulky and heavy unless they utilize smaller, simpler collimating lens, in which case the eye position from which the reticle can be seen without parallax is further restricted. The simple lens designs also tend to produce poor off-axis performance, which limits the size of the reticle that can be used.
A second type of reflex sight utilizes a tilted, off-axis spherical or parabolic reflector to collimate a point source from a light-emitting diode (LED). The reflector is spectrally coated to reflect only the red light of the LED and transmit the other wavelengths. This type of sight, commonly referred to as a "red dot sight", can only project a single dot due to the off-axis aberration of spherical or parabolic reflectors.
Laser designator sights are also utilized with small arms. A laser beam, usually from a laser diode, illuminates the target scene. The user positions the laser beam on the target to aim the weapon. This sight is undesirable in many applications since the laser emission could act as a targeting beacon for an adversary. The laser designator sight is also ineffective under many conditions. While laser radiation can maintain a small spot size over a long propagation distance, the brightness of the image of the spot scattered by the target surface reduces with range by a factor of 1/R.sup.2, where R is the target range. Thus, the laser light spot can be difficult to find under strong ambient light or at longer ranges. Also, the laser designator sight is ineffective in an application having a background, such as the sky, where there is no surface from which the laser beam would be scattered. Moreover, if there is more than one shooter using a laser designator sight, the multiple dots at the target plane can be confusing. Finally, laser designator sights are illegal for hunting in many states because they illuminate the target.
Holographic line sights for large weapons have also been developed. Once such sight is disclosed in U.S. Pat. No. 4,012,150, issued to Upatnieks. This holographic sight, however, is bulky, particularly since it requires a relatively large, stable wavelength laser.
A number of compact hologram displays have been developed, including the edge illuminated holograms disclosed in U.S. Pat. Nos. 4,643,515 and 5,151,800, both also issued to Upatnieks. These hologram displays have a compact monolithic structure. They also utilize a diffraction or reflection grating to minimize the effects of the wavelength drift of the laser diode light beam. These edge illuminated, monolithic holographic displays are, however, relatively expensive, and until now have not been incorporated into sights.